Implications of the non-observation of 6Li in halo stars for the primordial 7Li problem

Abstract

The primordial Lithium Problem is intimately connected to the assumption that the 7Li abundance observed in metal-poor halo stars is unchanged from its primordial value, which lies significantly below the predictions of standard big-bang nucleosynthesis. Two key lines of evidence have argued that these stars have not significantly depleted their initial (mostly primordial) 7Li: i) the lack of dispersion in Li abundance measurements at low metallicity (and high surface temperature); and ii) the detection of the more fragile 6Li isotope in at least two halo stars. The purported 6Li detections were in good agreement with predictions from cosmic-ray nucleosynthesis which is responsible for the origin of 6Li. This concordance left little room for 6Li depletion, and the apparent 6Li survival implied that 7Li largely evaded destruction, because stellar interiors destroy 6Li more vigorously then than 7Li. Recent (re)-observations of halo stars challenge the evidence against 7Li depletion: i) lithium elemental abundances now show significant dispersion, and ii) sensitive 6Li searches now reveal only upper limits to the 6Li/7Li ratio. We discuss the consequences of these 6Li non-detections on the primordial 7Li Problem, Galactic cosmic-ray nucleosynthesis, and the question of differential depletion of Li in stars. The tight new 6Li upper limits generally fall far below the predictions of cosmic-ray nucleosynthesis, implying that substantial 6Li depletion has occurred — by factors up to 50. We show that in stars with 6Li limits and thus lower bounds on 6Li depletion, an equal amount of 7Li depletion is more than sufficient to resolve the primordial 7Li Problem. This picture is consistent with well-studied stellar models in which 7Li is less depleted than 6Li, and strengthen the case that the Lithium Problem has an astrophysical solution. We conclude by suggesting future observations that could test these ideas.